EXACTLY how close have the last few elections been?

How was this calculated?

For each seat in each election the number of additional votes Lab/Con (depending on election) needed to win that seat by 1 vote was calculated. The seats were then ranked in order, with the most marginal seats first. The number of votes required was then tallied until it covered the number of seats that party would have needed to get a House of Commons majority of 1 seat (after having accounted for the Speaker and Sinn Fein).